You have to specifically request a cash advance. The bank is loaning you cash instead of credit (which in my opinion is all the same anyway... the bank is giving you something you don't have, and BoFA calls it a "cash credit") which leads to the high interest rate.
You're basically borrowing a CASH amount against the CREDIT you have. Let's say you're short on rent but can't pay rent with a credit card... that's where the cash advance comes from.
Again, unless you request it, the bank doesn't do anything with it.